Method of welding a metal piece to fine wires



Sept. 1969 w. H. DOUGLASS, 5R 3,466,419

METHOD OF WELDING A METAL PIECE To FINE WIRES Original Filed March 20.1964 INVENTOR. WALTER H. DOUGLAS-9.3K.

ATTORNEY United States Patent 3,466,419 METHOD OF WELDING A METAL PIECETO FINE WIRES Walter H. Douglass, Sr., Oreland, Pa., assignor to TRInc., a corporation of Ohio Original application Mar. 20, 1964, Ser. No.353,433, now Patent No. 3,302,156. Divided and this application May 21,1966, Ser. No. 565,361

Int Cl. B23k 9/28, 11/10 U.S. Cl. 219-91 4 Claims ABSTRACT OF THEDISCLOSURE This welding method is based on the use of the heat obtainedby the sudden rise in resistance between two closely spaced electrodeswhen the current between the electrodes is constant and the resistancesuddenly rises because of the inability of the conductor between theelectrodes to conduct high currents. To achieve this function, a metalpiece is placed over the wire to which it is to be welded, and a pair ofclosely spaced electrodes are placed on the metal piece over the wire.

The present invention relates to a method of welding a metal strip tofine wires and the article produced thereby. More particularly, thepresent invention relates to a method of welding a terminating metalstrip to a wire wound electrical resistance element.

This is a division of my co-pending application Ser. No. 353,433, filedMar. 20, 1964, now Patent No. 3,302,156.

In general, wire wound electrical resistors comprise a resistance wirehelically wound around and along a core of either an electricalinsulating material or a conductive material coated with an insulation,and conductive metal terminals secured to the ends of the wire. Theresistance wire is often very fine, such as between .010 in. to .0004in. in diameter, and may be of many alloys both ferrous and non-ferrous.However, the terminals are usually of a much heavier metal of adifferent material from that of the wire. The terminals must be attachedto the wire with a good electrical and mechanical bond. However, becauseof the composition of the metals of the wire and terminals and thedifierence in the masses thereof, it has been found to be extremelydiificult to form a good bond therebetween by conventional means, suchas brazing, soldering or other methods requiring high temperatures andthe use of fluxes which may destroy insulations or have a latentcorrosive action causing sudden unpredictable failure of the resistor.

A welded junction between the wire and terminal is most desirable, butnormally is impossible to obtain using standard welding techniques. Theheat and pressure required for welding plus the electrical and thermaldifferences existing between the fine resistance wires and the terminalmaterials nearly always prevents successful junction. It is essential tohave a thermal and conductive match to create controlled heating toeffect a resistance weld. To date the only way which welded junctions,for wire wound electrical resistors have been achieved has been bywelding a fine intermediate wire to a single turn of the resistance wireby percussion welding using high voltage without pressure, or byunwrapping a single turn of the resistance wire and welding it to asuitable junction material. The latter method is a tedious operationwhich is done under a microscope and requires delicate welding means.However, junctions or welds made by these techniques have been found tobe quite weak mechanically and have poor electrical characteristics forresistors. Also, these techniques are practically impossible to automateor mechanize 3,466,419 Patented Sept. 9, 1969 so that they must be doneby hand labor which is expensive.

It is an object of the present invention to provide a novel method ofwelding a metal piece to a fine wire.

It is another object of the present invention to provide a novel methodof welding a metal piece to one or more turns of a fine wire wound on acore.

It is still another object of the present invention to provide a methodof welding a metal terminal to one or more turns of a wire woundresistance element.

It is a further object of the present invention to provide a method ofwelding a metal piece to a fine wire which provides a good electricaland mechanical bond therebetween, which is relatively independent of thematerials of the metal piece and wire, and which can be automated.

It is a still further object of the present invention to provide a novelwelded junction between a metal piece and one or more turns of a finewire wound on a core.

It is yet another object of the present invention to provide a novelwelded junction between the terminal and the resistance wire of a wirewound electrical resistor.

Other objects will appear hereinafter.

The invention accordingly comprises the several steps and the relationof one or more of such steps with respect to each of the others, and thearticle possessing the features, properties, and the relation ofelements, which are exemplified in the following detailed disclosure,and the scope of the invention will be indicated in the claims.

For a fuller understanding of the nature and objects of the invention,reference should be had to the following detailed description taken inconnection with the accompanying drawings in which:

FIGURE 1 is a top plan view illustrating the first step of the method ofthe present invention.

FIGURE 2 is a sectional view taken along line 22 of FIGURE 1.

FIGURE 3 is a top plan view illustrating the second step of the methodof the present invention.

FIGURE 4 is a sectional view taken along line 44 of FIGURE 3.

FIGURE 5 is a top plan view illustrating the third step of the method ofthe present invention.

FIGURE 6 is a sectional view taken along line 66 of FIGURE 5.

FIGURE 7 is a top plan view illustrating the final step of the method ofthe present invention.

FIGURE 8 is a sectional view taken along line 8-8 of FIGURE 7.

In general, the welding method of the present invention is based on theuse of the heat obtained by the sudden rise in resistance between twoclosely spaced electrodes when the current between the electrodes isconstant and the resistance suddenly rises because of the inability ofthe conductor between the electrodes to conduct high currents. Toachieve this function, a metal piece is placed over the wire to which itis to be welded, and a pair of closely spaced electrodes are placed onthe metal piece over the wire. With the electrodes being properly spacedapart, an electrical current passing from one electrode to the otherthrough the metal piece will cause a window or slit to be blown out ofthe metal piece completely therethrough in the space between theelectrodes. With the portion of the metal piece between the electrodesremoved, the electrical potential between the electrodes exists throughthe wire beneath the metal piece. Since the wire is much finer than themetal piece, the wire cannot carry the current and will almostinstantaneously burn out. However, the heat from the current whichcauses the wire to burn out fuses the burnt out ends of the wire to fuseto the metal piece at the edges of the window or slit.

Thus, there is provided a welded junction between the wire and the metalpiece.

Referring to the drawing, FIGURES 1 and 2 illustrate the first step inthe method of the present invention. A flat strip 10 of an electricallyconductive metal is placed across and in contact with a plurality ofturns of a resistance wire 12 helically wound around a core 14. Asshown, the core 14 is of a metal and is coated with a film 16 of anelectrical insulating material, such as a varnish or plastic. However,the core 14 itself may be of an electrical insulating material.

As shown in FIGURES 3 and 4, a pair of electrodes 18a and 18b are thenplaced in closely spaced relation on the metal strip 10 over the turnsof the wire 12 which are to be welded to the strip. As shown in FIGURE3, the opposed surface of the electrodes 18a and 18b are preferably fiatand extend transversely across the turns of the wire 12. The electrodes18a and 18b are connected to a source of electrical current. As shown,the electrodes are connected across the secondary winding 20 of atransformer which is the output of a current source. When the currentsource is turned on, the current flows from one of the electrodes to theother through the metal strip 10. By properly spacing the electrodes 18aand 18b, the current passing through the portion of the metal strip 10between the electrodes, causes that portion of the metal strip toquickly heat up and a window or slit 22 (see FIG- URES and 6) is blownthrough the metal strip between the electrodes. As shown in FIGURE 5,the window 22 is of a width equal to the width of the electrodes. Withthe portion of the metal strip between the electrodes 18a and 18bremoved, the electrical potential between the electrodes then existsthrough the portions of the turns of the wire 12 extending across thewindow 22. Since the wire 12 is much finer than the metal strip 10, thewire cannot carry the current. Therefore the portions of the turns ofthe wire 12 extending across the window 22 burn out (see FIGURES 7 and8). The heat created in the wire 12 which causes the wire to burn out issufficient to fuse the ends 24a and 24b of the burnt out turns of thewire to the metal strip 10 at the edges of the window 22 (see FIGURE 8).Thus, the metal strip 10 is welded to the wire 12.

To achieve the welded junction of the present invention, the spacingbetween the electrodes 18a and 18b are relatively critical. The spacingbetween the electrodes 18a and 18b must be such that the current passingbetween the electrodes will create sufficient heat to burn out thewindow 22 in the metal strip 10 quickly enough to prevent the heat fromdamaging the insulation beneath the wire 12. If the electrodes arespaced too far apart, the time required to heat up the metal strip 10becomes too great causing damages to the insulation beneath the wire12.If the electrodes are too close together, the electrodes act as heatsinks drawing heat from the portion of the metal strip between theelectrodes. Thus, the time required to heat up the strip to form thewindow is so great as to cause damage to the insulation. It has beenfound that to achieve the welded junction of the present invention, thepreferred spacing between the electrodes is approximately the thicknessof the metal strip 10. With the electrodes properly spaced apart, thecurrent density necessary to achieve the welded junction of the presentinvention will vary depending on the thickness of the metal strip.However, the proper current density can be easily determinedexperimentally.

It has been found that the particular composition of the metal strip 10and the wire 12 has little, if any effect on the current densitynecessary to achieve the welded junction of the present invention. Forexample, using a current of approximately 320 amperes for a timeduration of approximately .008 second a nickel ribbon .003 inch thickand .042 inch wide was welded by the method of the present invention tothe various wires shown in the following table under an electrode forceof 8 to 10 ounces.

Resistance Wires Metal: Diameter (inches) Nickel-chromium alloy .00075Nickel-chromium alloy .00040 Nickel-chromium alloy .00175Nickel-chromium alloy .001 Nickel-chromium alloy .0013

Copper-Nickel alloy .00175 Copper-Nickel alloy .002 Cooper-Nickel alloy.0025 Copper-Nickel alloy .0045

- Using the same welding conditions, ribbons of a nickelchromium alloy,.042 inch wide and varying in thickness from .0005 inch to .003 inchwere welded to many of the wires shown in the above table.

The welded junction of the present invention has particular utility inattaching metal terminal strips to the ends of a wire wound resistanceelement for use in a variable resistor. Such a resistor comprises a wirewound on 'an elongated core of the type shown in the drawings, and aseparate metal terminal strip extending across and welded to a pluralityof the turns of the wire at each end of the core. The active portion ofthe resistance wire winding is the portion of the winding between theadjacent or innermost ends of the windows 22 in the terminal strips. Theother turns of the wire, which extend across each of the terminalstrips, tightly surround the core and are welded at each end to theterminal strip. Thus, there is provided a strong mechanical connectionbetween the terminal strips and the resistance element. When theresistance element is used in a variable resistor assembly, a movablecontact slidably engages the resistance wire winding along a linediametrically opposite the terminal strips so as to obtain variousresistance values. When the movable contact reaches the innermost end ofthe window in'the terminal strip, a zero resistance is obtained. Thezero resistance is also obtained when the contact engages all of theturns of the wire which are welded to the terminal strip. In mostvariable resistors, zero resistance is obtained only when the movablecontact engages the ends of the resistance wire so that the zeroresistance is attainable only by precise positioning of the movablecontact. However, with the welded junction of the present inventionwherein a zero resistance is provided over a plurality of turns of thewire, a zero resistance setting is easily attainable. Thus, the weldedjunction of the present invention provides a good electrical andmechanicalconnection between the terminal strips and the resistance wirewinding, and permits ease of obtaining a zero resistance setting.

'Although the welding method of the present invention isshown anddescribed for welding a flat metal strip to a wire winding, it can alsobe used to weld metal pieces of other shapes to the wire winding. Forexample, the method of the present invention can be used to weld to thewire winding metal caps or bands of the type shown in United StatesLetters Patent No. 1,763,772 issued June 17, 1930 to J. Geloso, entitledResistance and No. 2,019,457 issued Oct. 29, 1935 to E. G. Lodgeentitled Resistor.

Thus, there is provided by the present invention a method-of welding ametal piece to a winding of fine wire without damaging the core on whichthe wire is wound, and without high electrode pressures which woulddamage the wire. Also, the method of the present invention requires nospecial manual handling of the parts being welded so that it can beautomated. In addition, the present invention provides a welded junctionbetween a metal piece and one or more turns of a fine wire winding whichelectrically and mechanically are good and which is an excellentjunction for connecting metal terminals to a wire wound resistanceelement, particularly the resistance element of a variable resistor.

The present invention may be embodied in other specific forms withoutdeparting from the spirit or essential attributes thereof and,accordingly, reference should be made to the appended claims, ratherthan to the foregoing specification as indicating the scope of theinvention.

I claim:

1. A method of welding a metal piece to at least one turn of a fine wirewound around a core comprising the steps of placing the metal pieceacross at least one turn of the wire with one surface of the metal piececontacting the wire, placing a pair of closely spaced electrodes on asurface of the metal piece opposite the said surface with bothelectrodes crossing at least one turn of the wire and passing anelectrical current from one electrode to the other through the metalpiece of sufficient magnitude to burn a window out of the metal piecebetween the electrodes and fuse the wire to the edges of the window.

2. A method in accordance with claim 1 in which the spacing between theelectrodes is substantially equal to the thickness of the metal piece.

3. A method of welding a metal piece to a plurality of turns of a finewire wound around a core comprising the steps of placing the metal pieceover and in contact with a plurality of the turns of the wire Winding,placing a pair of closely spaced electrodes against the surface of themetal piece opposite the surface which contacts the Wire with theopposed surfaces of said electrodes extending across said turns of thewire winding, and passing an electrical current from one electrode tothe other through the metal piece of sufiicient magnitude to ibllll'l awindow out of the metal piece between the electrodes and fuse the turnsof the wire to the edges of the window.

4. The method in accordance with claim 3 in which the spacing betweenthe electrodes is substantially equal to the thickness of the metalpiece.

References Cited UNITED STATES PATENTS 2,279,316 4/1942 Herzog 219782,706,232 4/1955 Pilas 219--78 2,939,058 5/1960 Masterson 219-783,207,884 9/1965 Davis et a1. 219-86 JOSEPH V. TRUHE, Primary ExaminerL. H. BENDER, Assistant Examiner US. Cl. X.R. 219-78

